1. Field of the Invention
The present invention relates to a heat treatment apparatus that irradiates a substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display with light, to thereby heat the substrate.
2. Description of the Background Art
Short-time annealing has conventionally been achieved using rapid lamp annealers that use the energy of light emitted from halogen lamps to raise the temperature of a semiconductor wafer at rates of about several hundred degrees per second, as disclosed in U.S. Pat. No. 6,518,547. The rapid lamp annealers have shorter annealing time than heat treatment apparatuses that employ a resistance heating method using a kanthal heater or the like, but their annealing time is only on the order of several seconds.
On the other hand, flash lamp annealers that employ xenon flash lamps to irradiate a semiconductor wafer surface with a flash of light, as disclosed in U.S. Pat. No. 6,998,580, are known to achieve shorter annealing time. The length of time that the xenon flash lamps emit a flash of light is extremely short, about 10 milliseconds or less. The xenon flash lamps also have a spectral distribution of radiation that ranges from ultraviolet to near-infrared regions with shorter wavelengths than given with conventional halogen lamps, and that is in close agreement with a fundamental absorption band of a silicon semiconductor wafer. From this, the emission of a flash of light from the xenon flash lamps to a semiconductor wafer will produce only a small amount of transmitted light, thereby allowing a rapid rise in the temperature of the semiconductor wafer. It has also been found that an extremely-short-time (about 10 milliseconds or less) flash-light irradiation allows a selective rise in temperature only near the semiconductor wafer surface. For this reason, the flash lamp annealers are suitable for activating ions in an ion-implanted semiconductor wafer and allow the formation of shallow junctions by ion activation with no thermal diffusion of ions.
Laser annealers achieve still shorter annealing time than the flash lamp annealers. The laser annealers provide annealing by scanning a pulse laser beam with a duration of several dozen nanoseconds, in both X and Y directions.
There is, however, no conventional technique that provides annealing time within an intermediate range between the range achieved by the rapid lamp annealers using halogen lamps and the range achieved by the flash lamp annealers. In other words, there is no heat treatment apparatus that permits annealing time on the order of 10 milliseconds to 1 second at each place on the major surface of a semiconductor wafer. Nowadays, heat treatment providing such an intermediate rage of annealing time is being required in various process steps, such as activation, metallization, and wiring, in the manufacture of transistors.
The use of halogen lamps to achieve the above-described intermediate range of annealing time requires an increase in filament thickness due to the necessity of greater output and therefore has the drawback of increasing heat capacity and thereby rather slowing down the rates of temperature rise and fall.
The laser annealers, on the other hand, will theoretically achieve the above-described intermediate range of annealing time by increasing the length of time that a pulse laser beam remains at each place on a semiconductor wafer. But, keeping a pulse laser beam for a longer time at a particular place causes a temperature rise even in an unexposed region, so that the phenomenon of switching occurring in overlaps between temperature-raised regions becomes more pronounced. An even greater drawback is impractically low throughput because about one hour is necessary to process a single semiconductor wafer as a result of an increase in the length of time that a pulse laser beam remains at each place.
In addition, there are not only the simple need for rapid temperature rise and fall, but also the need to freely change the temperature profile at the time of annealing.